Impression Materials Lecture 4

Presented by:Dr. Hasan Md. RizviBDS (Dhaka Dental College), BCS(Health), FCPS (Orthodontics and Dentofacial Orthopedics) Lecturer, Dental unit. Shahid Shuruwardi Medical College.

Impression MaterialsLecture 4Classification of Impression MaterialsNon-elastic Materials1- Impression plaster (rigid)2- Impression Compound (thermoplastic)3- Impression waxes (thermoplastic)4-Impression paste (rigid)Elastic MaterialsI- Hydrocolloids II- Synthetic Elastomers

1- Reversible (Agar) 1- Polysulphides a Rubber base b Mercaptan c Thiokol 2- Irreversible (Alginate) 2- Polyether 3- Silicones a- Condensation silicone b- Addition cured silicone

Elastomeric Impression Materials A material that is used when an extremely accurate impression is essential. The term elastomeric means having elastic or rubberlike qualities.They are soft and rubber like and far more stronger and stable than the hydrocolloids.They are liquid polymers which can be converted to solid rubber at room temperature. By mixing with a suitable catalyst, they undergo polymerization and/or crosslinking reaction to produce a firm elastic solid.Initially they were called rubber-base or rubber impression materials. Currently, they referred to as elastomeric impression materials.

Types of Elastomeric Materials (chemically)PolysulfidePolyetherSilicone a- Condensation silicones b- Addition cured silicones (Polyvinyl siloxane )

Forms of Elastomeric Materials Light-bodiedAlso referred to as syringe type, or wash type. This material is used because of its ability to flow in and about the details of the prepared tooth. A special syringe, or extruder, is used to place the light-bodied material on and immediately around the prepared teeth.

Forms of Elastomeric Materials (cont)Regular and heavy-bodied or very heavy bodyOften referred to as traytype materials, they are much thicker. As the names imply, they are used to fill the tray. Their stiffness helps to force the lightbodied material into close contact with the prepared teeth and surrounding tissues to ensure a more accurate impression of the details of a preparation.

Non-aqueouselastomersPolyethersAdditionsiliconeCondensationsiliconePolysulfidesElastic

Uses of elastomeric impression materialsIn fixed partial dentures or fixed prosthesis for impressions of prepared teeth.Impressions of dentulous mouths for removable partial dentures.Impressions of edentulous mouths for complete dentures.Polyether is used for border molding of edentulous custom trays.For bite registration.Silicone duplicating material is used for making refractory casts during cast partial denture construction.

Supplied asCollapsible tubesLight and regular body material are also supplied in cartridges to be used with automixing tips and dispensers.Putty consistent supplied in jars

Polysulfide Impression MaterialThe first elastomeric impression material to be introduced (1950). It is also known as Mercaptan or Thiokol. Comersial name= coe-flex(lead dioxide system) Omni flex(Copper hydrooxide system)

Chemical makeup:Base paste: liquid polysulfied polymer 80-85% + fillers (Zinc sulfate, lithopone, or calcium sulfate dihydrate)

Reactor paste:Catalysts: lead dioxide 60-68%, Dibutyle phthalate 30-35%, Copper hydroxides, zinc peroxide, organic hydroperoxide. Cross-linking agent: Sulfur and/or lead peroxide.Other subctance: Magnesium stearate (retarder) + deodorantsTray adhesive :Butyl rubber or styrene dissolved in a \ solvent such as chloroform or a ketone is used with polysulfide.

Setting Reaction pbo2 +SMercaptan + Lead dioxide polysulfide + H2O(liquid polymer) (solid highly elastic flexible)

Exothermic reaction accelerated by moisture and heat

Manipulation and Technique Considerations for Polysulfide Material Dispense pastes at the top of the mixing pad. Mix pastes with the tip of a spatula to incorporate the material first.Transfer the material to the fresh surface of the mixing pad. Water, saliva, and blood affect polysulfide material. Impression should be removed quickly after setting-do not rock the tray. Adhesive must be thin and dry before adding the impression material. Wait 20 to 30 minutes before pouring the impression for the stress relaxation to occur in the material.Be careful of glove powder contamination of the impression. PROPERTIESAdvantages:Accurate if poured without delay (maximum storage time is 48 hours) Long working time (multiple preparations cases)Excellent tear resistance (high tensile strain before tearing)It has good flexibility (7%) and low hardness. A 2 mm spacing in the tray is sufficient for making an impression.It can be electroplated. The shelf life is good (2 years).

PROPERTIESDisadvantages:Unpleasant odor and color. Is messy to work with as these materials are extremely viscous and sticky. mixing is difficult. The mixing time is 45 seconds.It has a long setting time of 12.5 minutes (at 37C). In colder climates setting can take as long as 16 minutes. This adds to the patient's discomfort.curing shrinkage is high (0.45%) and continueseven after setting has the highest permanent deformation (3 to 5%) among the elastomers. Elastic recovery improves with time and so pouring of themodel should be delayed by half an hour. Further delay is avoided to minimize curing shrinkage.

SILICONE RUBBER IMPRESSION MATERIALSThese materials were developed to overcome some of the disadvantages of polysulfide materials.

This was earlier of two silicone impression materials----- refered to as conventional silicones.Supplied as:Collapsible tubes base tube being larger.Putty single large jar catalyst paste/liquid.Light /medium bodied , puttyCommercial name: sil 21. coltex, dent-a-scon

Condensation siliconesChemical makeup:

Base paste: polydimethyl siloxane + filler (metal oxide) 35-75% + coloring agent

Reactor paste:Catalysts: Stannous octateCross-linking agent: Orthoethyl silicate

Setting ReactionA codensation polymerization(exothermic) reaction occurs ---leading to crosslinkage between Orthoethyl silicate and the terminal hydroxy group of dimethyl siloxane to form three dimensional network. Stannous octateDimethyl siloxane + Orthoethyl silicate Silicone rubber + Ethyl alcohol ethyl alcohol formed as a by-product evaporates gradually from the set rubber leading to shrinkage. Tray adhesiveThe adhesive for silicones contain poly (dimethyl siloxane)(bonds with the rubber) and ethyl silicate (create a physical bond with the tray). PROPERTIESAdvantages:Pleasant color and odor. Although nontoxic, direct skin contact should be avoided to prevent any allergic reactions.Setting time is 6 to 9 minutes. Mixing time is 45 seconds.Excellent reproduction of surface details.Can be plated with silver/copper. Silver-plating is preferred.

PROPERTIESDisadvantages:Dimensional stability is comparatively less because of the high curing shrinkage (0.4 to 0.6%), and shrinkage due to evaporation of the ethyl alcohol byproduct. To avoid this the cast should be poured immediately. The permanent deformation is also high(1-3%).Tear strength (3000) gm/cm is lower than the polysulfides.It is stiffer and harder than polysulfide. The hardness increases with time.The spacing in the tray is increased to 3 mm to compensate for the stiffness.

Manipulation and Technique Considerations for Silicone Material The material has a limited shelf life. The tray requires a special tray adhesive. No syneresis or imbibition but does respond with shrinkage over time. The material is more flexible, so there is more chance for distortion during removal. Wait 20 to 30 minutes before pouring of models for stress relaxation to occur.

Disadvantages:

It is hydrophobic. The field should be well-dried before making an impression.Care should also be taken while pouring the cast to avoid air entrapment.Shelf life is slightly less than polysulfides due to the unstable nature of the orthoethyl silicates.

Addition silicones (polyvinyl siloxane)These new materials had bettter properties when compared to the condensation silicones. Also known as polyvinyl siloxane. Currently, are perhaps the most widely used elastomeric impression material worldwide.Commercial names: Reprosil (Dentsply) , President (Coltene), Provil.Supplied as:The base and catalyst pastes come in equal sized tubes.The different viscosities usually come in different colors like orange, blue, green, etc.

Supplied as:

Cartridge form with automixing tips For use with a dispensing gun. Putty jars Two equal sized plastic jarscontaining the base and catalyst A larger electric driven autodispenser and mixing device is also available(PentamixESPE). This machine stores larger quantities. At the press of the button, it dispenses and mixes the material. Available in Four Viscosities Light bodied/Medium bodied/Heavy bodied/Putty

COMPOSITIONBase:Poly (methyl hydrogen siloxane) or Other siloxane prepolymersFillersAccelerator:Divinyl polysiloxane Other siloxane prepolymersPlatinum salt-catalyst (chloroplatinic acid)Palladium or hydrogen absorberRetardersFillers

Setting ReactionIt is an addition reaction. In this case, the base polymer is terminated with vinyl groups and is crosslinked with silane (hydride groups). The reaction is activated by the platinum salt. Pt saltVinyl siloxane +Silane siloxaneSilicone rubberNo byproducts as long as vinyl= silane if unbalanced H2 is produced cause air bubble in stone model palladium added to absorb H2 nb sulfer retard setting reaction contamination can occur from latex gloves so use vinyl gloves.PROPERTIESAdvantages:Pleasant odor and color.Excellent reproduction of surface details.Setting time ranges from 5 to 9 minutes. Mixing time is 45 seconds.It has the best dimensional stability among the elastomers. It has a low curing shrinkage (0.17%) and the lowest permanent deformation (0.05 to 0.3%). If hydrogen gas is liberated pouring of stone should be delayed by 1-2 hours. Otherwise air bubbles will result.It has good tear strength (3000 gm/cm).It can be electroplated with silver or copper. However, hydrophilic silicones are more difficult to electroplate because of the surfactant added.Shelf life ranges from 1 to 2 years.

PROPERTIESDisadvantages:This may also cause allergic reaction so direct skin contact should be avoided.It is extremely hydrophobic, so similar care should be taken while making the impression and pouring the wet stone. Some manufacturers add a surfactant (detergent) to make it more hydrophilic.It has low flexibility and is harder than polysulfides. Extra spacing (3 mm) should be provided in the impression tray. Care should also be taken whileremoving the stone cast from the impression to avoid any breakage.

Manipulation and Technique Considerations for Polyvinyl Siloxane Material

For dimensional stability, this is the best impression material. Pouring of the model can be delayed up to 7 to 10 days. Stiffness of the material makes removal of the tray difficult. Material dispensed using auto-mixing unit and mixing tips.

One step impression technique

Two stage putty wash technique

POLYETHER RUBBER IMPRESSION MATERIAL

Polyether was introduced in Germany in the late 1960s. It has good mechanical properties and dimensional stability. Its disadvantage was that the working time was short and the material was very stiff. It is also expensive.Supplied as:Available as base and accelerator in collapsible tubes. The accelerator tube is usually smaller. A third tube containing a thinner was provided. Now it is available in three viscosities:Light bodiedMedium bodiedHeavy bodied

Supplied as:

Commercial name: Impregum F(premier),Ramitec, polyjel(Dentsply)

COMPOSITIONBase:Polyether polymer Colloidal silica --------fillerGlycolether or phthalate-------plasticizer

Accelerator Paste:Aromatic sulfonate ester ----cross-linking agentColloidal silica -----------fillerPhthalate or glycolether------plasticizer

SETTING REACTIONIt is cured by the reaction between aziridine rings which are at the end of branched polyether molecule. Crosslinking is brought about by the aromatic sulfonate ester via the imine end groups. The reaction is exothermic (4 to 5C).

Polyether + Sulfonic ester

Crosslinked rubber

PROPERTIESAdvantages:Pleasant odor and taste.Dimensional stability is very good. Curing shrinkage is low (0.24%)permanent deformation is also low (0.8 to 1.6%). However, polyethers absorb water and can change dimension. Therefore prolonged storage in humid climates is not recommended.Tear strength is good (3000 gm/cm).It is hydrophilic, so moisture in the impression field is not so critical the best compatibility with stone among the elastomers.It can be electroplated with silver or copper.The shelf life is excellent more than 2 years.It has excellent detail reproduction (20 micron).

PROPERTIESDisadvantages:The sulfonic ester can cause skin reactions. Thorough mixing is recommended before making an impression and direct skin contact should be avoided.Setting time is around 6 to 8 minutes. Mixing should be done quickly which is 30 seconds. Heat decreases the setting time.It is extremely stiff,(flexibility 3%). It is harder than,polysulfides and increases with time. Removing it from undercuts can be difficult, so additional spacing of(4 mm) is recommended. Care should also be taken while removing the cast from the impression to avoid any breakage.Prolonged exposure may cause the surface of the impression to become sticky (depolymerizes) after several days

Manipulation of rubber impression materialsPUTTYWith addition silicones, equal scoops of base and accelerator are taken. With condensation silicones, the required number of scoops of base and recommended amount of liquid or paste accelerator is taken. In either case mixing is done by kneading between the fingers. Mixing is done until a streak free mix is obtained.PASTEFor Polysulfides and Addition SiliconesEqual lengths of base and accelerator is extruded onto the mixing pad side-by-side without touching. The accelerator paste is then incorporated into the base paste. Mixing is done using a tapered stiff bladed spatula. Just before loading the tray the material should be spread in a thin layer to release the trapped air bubbles. A streak free mix is obtained in 45 seconds.Manipulation of rubber impression materialsCondensation SiliconsUnlike addition silicone, the quantity of catalyst paste needed is very little. The manufacturer usually marks the length required on the mixing pad. The two pastes therefore are of unequal length and diameter For PolyetherThe required amount of thinner (when supplied) may be added to the base and accelerator depending on the viscosity needed. Again, like condensation silicone. the quantity of accelerator needed is very little. The ratio is usually displayed on the mixing pad. The mixing should be done quickly. The mixing time is 30 seoconds

TECHNICAL CONSIDERATIONSImpressions can be made in custom or stock trays.Elastomers do not adhere well to the tray. An adhesive should be applied to the tray and allowed to dry before making impressions. Theadhesive cements provided with the various elastomers are not interchangeable. A slightly roughened tray surface will increase the adhesion.For putty impressions, a perforated stock tray is used. The perforations help to retain the putty in the tray .

TECHNICAL CONSIDERATIONSThe spacing given is between 2 to 4 mm.METHODS OF MAKING IMPRESSIONS:SINGLE MIX TECHNIQUETray used Resin custom tray with 2-4 mm spacing.Viscosity used Regular only.Method : The paste is mixed and part of it is loaded on to the tray and part into a syringe. The syringe material is then injected on to the prepared area of impression. The tray with material is seated over it. The material is allowed to set.

METHODS OF MAKING IMPRESSIONS:

MULTIPLE MIX TECHNIQUETray used Resin custom tray with 2-4 mm spacing.Viscosity used (a) Heavy bodied and (b) light bodied.Method : The two viscosities are mixed simultaneously on separate pads.heavy body is loaded into the tray while the light bodied is loaded into the syringe. The syringe material is injected over the preparation. The tray containing the heavy body if then seated over it. Both materials set together toproduce a single impression.

METHODS OF MAKING IMPRESSIONS:

ONE-STAGE PUTTY-WASH TECHNIQUE shownTWO-STAGE PUTTY-WASH TECHNIQUEshownREMOVAL OF THE IMPRESSIONRemoval is facilitated by breaking the airseal. This can be done by teasing the borders of the tray parallel to the path of insertion until the air leaks into the tray.In addition to the holding the tray handle, a finger on the buccal portion of the tray may be used to apply additional pressure to dislodge the tray.

INFECTION CONTROL

Rubber impression materials are disinfected by immersing in disinfectant solutions. 10 minutes in 2% glutaraldehyde or 3 minutes in chlorine dioxide solutions have been found to be satisfactory. Because of its tendency to absorb water, a spray of chlorine dioxide is preferred in case of polyether. Other disinfectants used are phenol and iodophor.

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